The invention relates to stair-climbing hand trucks or dollies which are powered for raising and lowering heavy loads.
Various hand trucks are known which are structured for facilitating the movement of a heavy load up and down stairs, over curbs and the like, and onto and off of an elevated platform. Typical stair-climbing hand trucks are disclosed in U.S. Pat. Nos. 2,192,396, 2,904,201, and 3,907,138. Such hand trucks typically comprise a main frame member and a secondary frame member which are telescopically or otherwise slidably mounted so that the secondary frame can move upward and downward on the main frame. The load to be moved is supported on one frame and the dolly wheels are supported on the other. Then, the load can be raised or lowered with respect to the wheels which are fixed on the edge of a step or platform. Alternatively, the frame member supporting the wheels can be raised or lowered with respect to the load-supporting member resting on a step, thereby hooking the wheels over a higher or lower step. By alternately carrying out these two relative motions of the frame members, a load can be manipulated over otherwise insurmountable obstacles.
Known hand trucks are commonly powered by an electric motor to enable relative movement of the two frame members even when a large, heavy and bulky load is supported thereon. Although the motor enables one to raise and lower a large bulky load, such loads are nevertheless difficult to maneuver and wheel around and require delicate balance and constant attention of the operator. Furthermore, the frame members themselves must be made of a heavier steel construction to support the increased weight made possible by the use of an electric motor. The unloaded hand truck by itself is thus significantly heavier, which only adds to the care which must be observed when manipulating a bulky load.
In addition, problems have been experienced with the electric motors used to power stair-climbing hand trucks. If the motor is not powerful enough, it will burn out when the operator attempts to raise too heavy a load. Since hand truck operators do not generally weigh their intended cargoes before loading onto a hand truck, burnout is the inevitable fate of such electric motors. Attempts have been made to counter this problem by installing motors powerful enough to raise any load within reason. Motors such as these, however, tend to cause jamming or damage to structural elements of the hand truck when the members are fully extended or fully retracted and the motor is not disengaged in time. When manipulating bulky, heavy objects in confined spaces, a hand truck operator is frequently unable to see when the frame members are becoming dangerously overextended.
The difficulty of maneuvering heavy loads gracefully, which is exacerbated by the weight of the unloaded hand truck itself, and the possibility of damage to the hand truck from over extension or burnout of the motor while a heavy load is being maneuvered presents a serious safety hazard to the hand truck operator.